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PostPosted: Thu Nov 01, 2012 1:39 am 
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Brazilian Rosewood
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I'm trying my first guitar using a hardwood top. I feel conflicted as to an approach. One would assume that knowing deflection in wood 'A', say sitka, yields a successful guitar, then using the same deflection in any other type of wood should yield safe results. I'm more concerned with implosion prevention than super awesome tone for the first try. Am I right in thinking that a certain known amount of stiffness in the top will be safe regardless of the wood species? Or are there other properties that need to be taken into account that I am not considering?

Secondly, the top I am using is really stiff across the grain compared to spruce, nearly equal. But I only have historical data for long grain deflection. To account for this, can I just skew the tone bars more towards long grain direction to make it more flexible cross grain?

Thanks


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PostPosted: Thu Nov 01, 2012 8:22 am 
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The deflection idea sounds reasonable since that's how some of us do tops with various types of spruce. That's probably how I would approach it, however, I haven't tried hardwood so would be good to hear from someone who has some experience.

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PostPosted: Thu Nov 01, 2012 9:29 am 
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Brazilian Rosewood
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Seems reasonable right? X amount of deflection equals x amount of strength, regardless of what kind of wood. Let's here from some more folks...


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PostPosted: Thu Nov 01, 2012 10:37 am 
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meddlingfool wrote:
Seems reasonable right? X amount of deflection equals x amount of strength, regardless of what kind of wood. Let's here from some more folks...


Not quite, deflection is a measurement of stiffness, not strength. I don't suspect you'll have any problems though.

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PostPosted: Thu Nov 01, 2012 11:53 am 
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Brazilian Rosewood
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Perhaps resistance to load would be more accurate, but what I mean is 'ability to handle string tension without the bridge rolling forward and collapsing into a black hole', whatever the technical term for that might be....


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PostPosted: Thu Nov 01, 2012 1:58 pm 
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Brazilian Rosewood
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...and whether you say 'stiffness' or 'resistance to load', it's the thing that limits how thin and light you can make the top.

The problem on the guitar is that the strings have to be above the bending plane of the top. The woods we use could more than handle the string loads in direct tension and compression, even without bracing, but the bridge torque bends the top, and then it 'cold creeps'; bellies and sinks.

I think you'll find if you test a lot of samples that most hardwoods are not all that much stiffer at a given thickness than the 'usual suspect' softwoods, and the soft woods will be a lot less dense. It's simply a function of the structure of the material. In the end you'll most likely have to leave the hardwood tops about as thick as you would have made a good spruce top, and with something like mahogany the weight will be one and a half or two times greater. This might not be a bad thing: if you're making an acoustic/electric that's meant to be played plugged in the added weight on the top can help limit feedback. Also, give similar stiffness for greater mass, the hardwood top may well end up with lower resonant pitches, which might help if you're making a bass. When given lemons...


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PostPosted: Thu Nov 01, 2012 2:22 pm 
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This particular piece is not as strong as spruce, needing to be .125-.129 to give the same deflection as spruce. I actually skimmed a little off the top to give it just a hair more flexibility than I would with spruce, figuring I can make it up with the bracing and achieve a touch of weight saving.
This particular piece has a density of .51, so not that much heavier than dense spruce. What it will sound like is TBD...
I think it's the cold creep that's got me edgy. I'm wondering, is measuring the deflection and using safe measurements for spruce really going to translate into a different species. Logic says yes, unless there is a measurable property in wood that would indicate colld creep that I'm not aware of....
I know primavera has been used successfully for flamencos....


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PostPosted: Thu Nov 01, 2012 5:02 pm 
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A method of determining how far to go when thinning tops is written up in the usual place. Way too much to report in detail here, but it takes into account long, cross and shear moduli for the piece of wood you have, which, together with its density, gives a dimension to thin it to to get the same uncoupled vibrational performance (frequency of vibration of the un-braced plate) irrespective of the material properties and the size of the guitar. You may find that there is some overlap between the properties of spruce and mahogany at the extremes of their ranges and that is why using a technique like this is important. You can't hope to get wood of the same properties every time, even out of the same tree, so unless you have a technique to deal with the normal variation, your end result can be fairly random.

Regarding cold creep, one property that gives a good indication of how susceptible a species is to cold creep is how easy it is to heat bend. The easily heat bendable woods are most susceptible to cold creep. The more and wider the humidity cycling, temperature cycling and the higher the static stress (as a proportion of yield) the more cold creep you'll get. If you beneficially use carbon fibre in the structure, you'll reduce the tendency to cold creep. Designing and building with CF is also covered in the usual place. The propensity of a guitar top to cold creep is largely determined by the bracing, because the tops of the braces are where the stresses are highest.

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PostPosted: Thu Nov 01, 2012 5:18 pm 
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Brazilian Rosewood
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While the usual place is definitely a useful place, a great deal of it is writ in a language alien to me. I suspect Greek but I'm uncertain...
The summaries I have used to great result.
I like looking at the pictures too.


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PostPosted: Thu Nov 01, 2012 7:00 pm 
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Perseverance will be rewarded! Unfortunately there are no really simple answers or we'd all know them already.

If you can find a kid who's good with a spreadsheet, they'll have that knocked into shape for you in no time. There's quite a discussion group going on the ANZLF for people tackling the maths. The plate design stuff is quite straight forward. I think that all the main mathematical stuff has now been tested and it all seems to work for them, even optimisation for the intonation solutions and the 4-DOF model. Excel does seem to struggle with some of the maths, though, which needs to be coded up with ill-conditioning front and centre of your mind when you do it. The consensus seems to be that the maths as written is pretty accurate, which is gratifying to know, given the amount of it.

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PostPosted: Thu Nov 01, 2012 8:55 pm 
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Brazilian Rosewood
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Ah, so it's math! Thought it might have been Australian...;)

Actually, I've managed to use the usual place to get my guitars to where I'm pretty happy with them without understanding the math or being able to use it. Just wondering if what's good for the goose is good for the gander....


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PostPosted: Thu Nov 22, 2012 1:38 am 
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meddlingfool wrote:
Perhaps resistance to load would be more accurate, but what I mean is 'ability to handle string tension without the bridge rolling forward and collapsing into a black hole', whatever the technical term for that might be....

A lively top will not resist collapsing very well on its own, and is not very strong or thick. Most of the load is handled by the x brace and bridge plate.
I use the same deflection value for soft and hard wood tops with good sresults.

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PostPosted: Thu Nov 22, 2012 1:48 am 
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Just finished stringing up said guitar and it's a beauty, no sign if top distortion. Happy camper, and soon to be a happy customer...
Will repeat....


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PostPosted: Thu Nov 22, 2012 1:03 pm 
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Quote:
To account for this, can I just skew the tone bars more towards long grain direction to make it more flexible cross grain?

That would be my approach.

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PostPosted: Thu Nov 22, 2012 1:09 pm 
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Brazilian Rosewood
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Worked really well...


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PostPosted: Thu Nov 22, 2012 4:36 pm 
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What was the top made of? And how did the volume compare with spruce or cedar? Tone?

I'm intrigued because I come from ukulele making (hobby builder only), and here hardwood tops are around 1.8mm or less compared to 2.1mm for spruce and a little more for WRC.

I suspect, though, that the difference is to control tone, not stiffness/deflection/potential collapses. I made a spruce soprano with a top around 1.8mm and the sound was distinctly distorted. WRC tops at 2.2mm or so don't do this.

Hardwood tops need to be that thin to give the uke a reasonable volume of sound, but not so softwood (except yew, which I think needs to be in between those thicknesses).

I ask because I'm planning a spruce-topped concert as a friend's birthday present. He's a fan of Selmer jazz guitars but can't afford one, or a decent copy, so I'm working out a similar-styled uke. My thoughts were that for a jazz guitar influenced uke sound I should actually go for a thin spruce top, and deal with the tonal issues via the bracing. Maybe your experience in the other direction will help me decide.


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PostPosted: Thu Nov 22, 2012 5:08 pm 
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Brazilian Rosewood
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Hmmm....

Zero uke experience.

I ended up with a thicker top than I would have with spruce or cedar, because the primavera was not as stiff as them. I just thinned the top to the same deflection as I did with spruce and forged ahead, with the specific thickness being an artifact....


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